1. Field of the Invention
The present invention relates to an in-wheel motor, and particularly, to an in-wheel motor with high durability.
2. Description of the Background Art
A known conventional in-wheel motor drive scheme is to support a hollow motor by a motor suspension (for example, see WO02/083446 and Go Nagaya, Yasumichi Wakao and Akihiko Abe “Development of an In-Wheel Motor with Advanced Dynamic-Damper Mechanism”, the society of automotive engineers of Japan, Nov. 26, 2002, proceedings of conference No. 83-02, pp. 9-12). The hollow motor is coupled to a wheel of a wheel unit to rotate the wheel. The hollow motor is supported by the motor suspension so that it can vibrate in the vertical direction of the vehicle, and thus separated from the unsprung weight. The wheel is supported by a suspension arm to the vehicle. In this in-wheel motor drive scheme, when the wheel unit vibrates, the hollow motor receives the vibration of the wheel unit through the wheel and vibrates in the vertical direction of the vehicle. The vibration of the hollow motor cancels unsprung vibration.
Meanwhile, in order not to increase the volume of a motor, it is desirable to transmit output torque of the motor via a speed reducer to a wheel.
Accordingly, in a conventional in-wheel motor drive scheme, a geared motor is employed as the in-wheel motor, in which a hollow inner rotor type motor (motor) and a planetary speed reducer are integrally assembled in a motor case.
Specifically, the motor of the geared motor is a hollow inner rotor type motor, which includes a stator fixed to a non-rotary case provided on the outer side in the radial direction and a rotor fixed to a rotary case provided on the inner side in the radial direction. The non-rotary case is coupled to a knuckle that is a fixed portion, while the rotary case is coupled to a sun gear of the planetary speed reducer and rotatably connected to the shaft portion of the motor case.
When the sun gear rotates as the rotor rotates, the rotation speed of the sun gear is converted and reduced to a speed corresponding to the orbital period of the planetary gear, and transmitted from a carrier to the shaft coupled to the output shaft of the speed reducer. The shaft has a universal joint coupling the output shaft of the planetary speed reducer and the wheel.
The geared motor is float-mounted to unsprung weight corresponding components that are a components around each wheel, and the rotation axis of the motor can swing in the radial direction independently of the rotation axis of the wheel unit. Thus, the mass of the motor functions as the mass of a so-called dynamic damper to cancel unsprung vibration when the vehicle runs over a bad road. As a result, the riding comfort of the vehicle is improved.
Here, the motor and the planetary speed reducer in a conventional geared motor are arranged in the following order from the wheel toward the vehicle body: the universal joint, the motor, and the planetary speed reducer. Then, the vibration of the wheel unit is transmitted, via the wheel and the universal joint, from the motor to the planetary speed reducer. Here, stress due to vibration of the wheel unit is applied to each of the motor and the planetary speed reducer. As this stress is greater proportionate to the distance from the wheel unit that is the source of vibration, a relatively great stress is applied to the planetary speed reducer that is located farthest from the wheel unit.
As known well, the planetary speed reducer has a structure where a plurality of gears are engaged, and hence stiffness thereof to the stress is relatively low. Accordingly, there has been a problem with the conventional in-wheel motor drive scheme that the durability of the in-wheel motor is impaired.